1. Field of the Invention
This invention relates to tin oxide coatings, and more particularly, to an improved method of producing fluorine-doped tin oxide coatings by chemical vapor deposition.
2. Description of the Prior Art
Fluorine-doped tin oxide coatings are known to impart useful properties to many different substrate surfaces, including glass, ceramics, metals and elemental filaments. Such coated substrates find use as heat reflective elements, in energy efficient windows and in opto-electronic and semiconductor devices.
Chemical vapor deposition of tin oxide coatings has been described in the literature, See, for example, U.S. Pat. Nos. 3,677,814; 3,949,146; 4,130,673; 4,146,657; 4,213,594; 4,265,974; Thin Solid Films 77, 51-63 (1981); 65-66; and J. Electrochem. Soc. 122, 1144 (1975).
The chemical vapor deposition method described in the prior art suffers from one or more disadvantages in that the process conditions, or compositions used, or the properties of the resulting tin oxide coatings, are not entirely satisfactory for certain commercial applications. For example, where the product is an energy efficient window, success has been limited by the relatively high and variable sheet resistance of the coatings obtained in the process, and the low deposition rates.
Kane in U.S. Pat. No. 3,949,146 describes chemical vapor deposition of tin oxide coatings from dibutyltin diacetate, bis(tri-n-propyl)tin oxide or bis(tri-n-butyl)tin oxide doped with antimony. However, there is no disclosure of a method of producing a low sheet resistance fluorine-doped coating whose resistance is independent of process variables.
A chemical deposition method also is disclosed by Kato in U.S. Pat. No. 4,500,567. This process uses a gaseous mixture of monobutyltin trichloride and a halodifluoromethane which is formed from separate gaseous streams of each reactant. The gaseous mixture then is deposited onto a glass surface. However, in this method the electrical properties of the coating are not adequate, and the deposition times are long.
As will be apparent from the above-described review of the prior art, there is a need for an improved chemical vapor deposition process of making fluorine doped tin oxide coatings, particularly a process which will provide such coatings having a constant and minimum sheet resistance under a range of process conditions.